U.S. patent number 6,884,003 [Application Number 10/616,400] was granted by the patent office on 2005-04-26 for multi-cellular floating platform with central riser buoy.
This patent grant is currently assigned to Deepwater Technologies, Inc.. Invention is credited to Edward E. Horton, III.
United States Patent |
6,884,003 |
Horton, III |
April 26, 2005 |
Multi-cellular floating platform with central riser buoy
Abstract
A semi-submersible floating platform for offshore drilling
and/or production of petroleum product from the seabed includes a
base having a first moon pool; a plurality of vertical outer
buoyancy columns extending upwardly from the base; a deck structure
supported by the buoyancy columns and having a second moon pool; a
central columnar buoyancy apparatus having a lower portion guided
within the first moon pool and an upper portion guided within the
second moon pool; and at least one vertical riser passing through
the buoyancy apparatus. Each riser has a lower portion that is
horizontally restrained within the buoyancy apparatus below the
center of gravity thereof. In a preferred embodiment, the platform
includes at least two vertical risers attached to a single buoyancy
apparatus.
Inventors: |
Horton, III; Edward E.
(Houston, TX) |
Assignee: |
Deepwater Technologies, Inc.
(Houston, TX)
|
Family
ID: |
33514286 |
Appl.
No.: |
10/616,400 |
Filed: |
July 9, 2003 |
Current U.S.
Class: |
405/224.2;
405/223.1; 405/224 |
Current CPC
Class: |
B63B
77/00 (20200101); E21B 19/004 (20130101); B63B
35/4413 (20130101) |
Current International
Class: |
B63B
9/00 (20060101); B63B 9/06 (20060101); B63B
35/44 (20060101); E02B 17/02 (20060101); E21B
19/00 (20060101); E02B 17/00 (20060101); B63B
035/44 () |
Field of
Search: |
;405/223.1,224,224.2,224.4 ;114/264,265 ;166/355 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Singh; Sunil
Attorney, Agent or Firm: Klein, O'Neill & Singh, LLP
Klein; Howard J.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit, under 35 U.S.C. Section
119(e), of co-pending U.S. provisional application No. 60/478,870;
filed Jun. 16, 2003.
Claims
What is claimed is:
1. A semi-submersible platform, comprising: a base having a first
moon pool; a plurality of vertical outer buoyancy columns extending
upwardly from the base; a deck structure supported by the buoyancy
columns and having a second moon pool; a central columnar buoyancy
apparatus that is guided within the first and second moon pools for
vertical movement between an upper position and a lower position
relative to the base and the deck structure; an upper stop assembly
on the buoyancy apparatus that is engageable against the deck
structure when the buoyancy apparatus is in its upper position; a
lower stop assembly on the buoyancy apparatus that is engageable
against the base when the buoyancy apparatus is in its lower
position; and a riser passing through the buoyancy apparatus and
horizontally restrained within the buoyancy apparatus below the
center of gravity thereof.
2. The semi-submersible platform of claim 1, wherein at least two
vertical risers pass through the central columnar buoyancy
apparatus and are horizontally restrained below the center of
gravity thereof.
3. The semi-submersible platform of claim 1, wherein the base is
buoyant.
4. The semi-submersible platform of claim 1, wherein the central
columnar buoyancy apparatus has a lower portion, and wherein the
riser is attached to the central columnar buoyancy apparatus within
the lower portion thereof.
5. The semi-submersible platform of claim 4, wherein the central
columnar buoyancy apparatus has an upper portion, and wherein the
riser is attached to the buoyancy apparatus within the upper
portion thereof.
6. The semi-submersible platform of claim 1, wherein the central
columnar buoyancy apparatus comprises multiple compartments.
7. The semi-submersible platform of claim 1, wherein the central
columnar buoyancy apparatus is guided within each of the first and
second moon pools by a plurality of guide assemblies.
8. The semi-submersible platform of claim 7, wherein the guide
assemblies are complaint.
9. The semi-submersible platform of claim 7, wherein the guide
assemblies maintain substantially constant contact with the central
columnar buoyancy apparatus.
10. The semi-submersible platform of claim 7, wherein each of the
guide assemblies includes a wear pad that engages the central
columnar buoyancy apparatus.
11. The semi-submersible platform of claim 7, wherein each of the
guide assemblies includes a roller that engages the central
columnar buoyancy apparatus.
12. The semi-submersible platform of claim 11, wherein the central
buoyancy apparatus includes a plurality of vertical rails on the
periphery thereof, each of the rails being positioned for
engagement by one of the rollers.
13. The semi-submersible platform of claim 7, wherein the guide
assemblies include a plurality of wear pads on the periphery of the
central columnar buoyancy apparatus.
14. The semi-submersible platform of claim 7, wherein each of the
guide assemblies comprises a guide module that is lockably
installable within one of the moon pools.
15. The semi-submersible platform of claim 1, wherein the buoyancy
apparatus includes structure that defines an internal moon
pool.
16. The semi-submersible platform of claim 1, wherein the platform
includes a well deck that is supported by the buoyancy
apparatus.
17. A method of installing a floating, semi-submersible platform at
an operational site on the sea surface over the seabed, comprising
the steps of: (a) providing an assembly comprising a buoyant base
having a plurality vertical outer buoyancy columns upwardly
therefrom, and a central columnar buoyancy apparatus located
centrally within the base, the central columnar buoyancy apparatus
being movable vertically relative to the base between an upper
position and a lower position; (b) towing the assembly at a shallow
draft to a first site with the central columnar buoyancy apparatus
in its upper position; (c) ballasting down the central columnar
buoyancy apparatus to its lower position; (d) ballasting down the
base to a first draft such that the outer buoyancy columns extend
just above the sea surface; (e) floating a deck structure over the
base, the outer buoyancy columns, and the central columnar buoy;
(f) deballasting the outer columns to lift the deck structure; (g)
deballasting the central columnar buoyancy apparatus to raise it to
its upper position in which it engages the deck structure to form a
platform; (h) towing the platform to a second site at an
intermediate draft; (i) ballasting down the platform to an
operational draft; and (j) anchoring the platform to the
seabed.
18. The method of claim 17, wherein the central columnar buoyancy
apparatus includes an upper stop assembly and a lower stop
assembly, and wherein the step of ballasting down the buoyancy
apparatus is performed until the lower stop assembly abuts against
the base, and wherein the step of deballasting the buoyancy
apparatus is performed until the upper stop assembly abuts against
the deck structure.
Description
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
BACKGROUND OF THE INVENTION
The present invention relates to offshore platforms, and
specifically to offshore platforms designed for dry tree
applications. More particularly, the present invention relates to a
new production and/or drilling riser system used in deep draft
semi-submersible platforms.
Conventional dry tree offshore platforms are low heave floating
platforms, such as spars, TLPs (Tension Leg Platforms), and deep
draft semi-submersible platforms. These platforms are able to
support a plurality of vertical production and/or drilling risers.
These platforms may comprise a well deck, where the surface trees
(arranged on top of the riser) will be located, and a production
deck where all the crude oil will be manifolded and sent to a
processing facility to separate water, oil and gas. In conventional
dry tree offshore platforms, vertical risers running from the well
head to the well deck are supported by a tensioning apparatus.
These vertical risers are called Top Tensioned Risers (TTRs).
One prior art TTR design uses active hydraulic tensioners to
independently support the risers. Each riser extends vertically
from the wellhead to the well deck of the offshore platform. The
riser is supported by active hydraulic cylinders connected to the
well deck of the offshore platform, allowing the platform to move
up and down relative to the risers and thus partially isolating the
risers from the heave motions of the hull. A surface tree is
connected on top of the riser, and a high pressure flexible jumper
connects the surface tree to the production deck. As tension and
stroke requirements increase, these active tensioners become
prohibitively expensive. Furthermore, the loads have to be
supported by the offshore platform.
A second prior art design uses passive buoyancy cans to
independently support the risers. Each riser extends vertically
from the wellhead to the well deck of the offshore platform. The
riser passes from the wellhead through the keel of the floating
platform into a stem pipe, on which buoyancy cans are attached.
This stem pipe extends above the buoyancy cans and supports the
platform to which the riser and the surface tree are attached. A
high pressure flexible jumper connects the surface tree to the
production deck. Because the risers are independently supported by
the buoyancy cans (relative to the hull), the hull is able to move
up and down relative to the risers, and thus the risers are
isolated from the heave motions of the offshore platform. The
buoyancy cans need to provide enough buoyancy to support the
required top tension in the risers, the weight of the can and the
stem pipe, and the weight of the surface tree. With increased
depth, the buoyancy required to support the riser system will also
increase, thereby requiring larger buoyancy cans. Consequently the
deck space required to accommodate all the risers will increase.
Designing and manufacturing individual buoyancy cans for each riser
is also costly.
Offshore environmental conditions are often harsh. Actions of wind,
waves and currents on an offshore structure can have severe
effects, especially in the layer of the sea between the surface and
a depth of about 150-300 ft. (about 45 m to about 90 m) which is
called the "splash zone". These actions attenuate with the water
depth. In deep draft semi-submersible platforms, the vertical
risers are subjected to the effects of high waves and current
forces near the surface, which puts strain on the risers and can
lead to VIV (Vortex Induced Vibrations). Consequently, in both of
the aforementioned designs, each riser must be provided with
strakes to prevent or minimize VIV, thereby increasing
manufacturing costs.
A third prior art design, exemplified by U.S. Pat. Nos. 5,439,321
and 4,913,238, proposes to connect all the TTRs to a single
(independent from the work platform) buoyancy apparatus in order to
create a kind of small well deck TLP (Tension Leg Platform) to be
received in a conventional semi-submersible platform. The small
well deck TLP will be anchored with tendons connected to the outer
periphery of the buoyancy apparatus. The well deck TLP is not
dependent from the floating platform. In the apparatus disclosed in
U.S. Pat. No. 5,439,321 the well deck TLP is connected to the
floating platform through a cross springs mooring system, and in
the apparatus disclosed in U.S. Pat. No. 4,913,238, through
centralizer dollies arranged at the bottom of the floating
platform. This device restrains the TLP partially horizontally;
however the TLP is still able to rotate relative to the platform.
The well deck TLP through this anchoring system has very good
motion characteristics; however the conventional semi-submersible
platform has large motions which will be transmitted to the well
deck TLP, and the tendon and riser system must be designed to
withstand these horizontal and pitch motions as well as large
impact loads between the two floating vessels. Furthermore, as the
conventional semi-submersible platform undergoes large motions,
long, flexible jumpers to carry crude oil from the well deck TLP to
the production deck on the semi-submersible platform are required
to absorb the large relative motions between the two vessels.
Finally, the vertical risers are connected only in the upper part
of the single buoyancy apparatus. Nothing is proposed for
horizontal restraint of the motion of the risers within the
buoy.
SUMMARY OF THE INVENTION
The present invention addresses the problems just described and
proposes a new passive tensioning system for Top Tensioned Risers
in a deep draft semi-submersible platform.
In a first aspect, the present invention is a deep draft
semi-submersible platform for drilling and/or production, the
floating platform comprising: a base having a first moon pool; a
plurality of buoyant vertical support columns arranged on the base;
a deck structure supported by the columns and having a second moon
pool; and a riser system comprising a single buoyancy apparatus
having upper and lower parts, supporting at least two vertical
risers; wherein the single buoyancy apparatus is guided at a lower
location by the first moon pool and at an upper location by the
second moon pool; and wherein the vertical risers are attached to
the single buoyancy apparatus in the upper part of the buoyancy
apparatus and are at least horizontally restrained in the lower
part of the buoyancy apparatus.
In a second aspect, the present invention is a method for
installing a floating deep draft semi-submersible platform
comprising the following steps: (a) providing an assembly
comprising a buoyant base having a plurality of vertical outer
buoyancy columns extending upwardly therefrom, and a central
columnar buoyancy apparatus guided centrally within the base, the
central columnar buoyancy apparatus being movable vertically
relative to the base between an upper position and a lower
position; (b) towing the assembly at a shallow draft to a first
site with the central columnar buoyancy apparatus in its upper
position; (c) ballasting down the central columnar buoyancy
apparatus to its lower position; (d) ballasting down the base to a
first draft such that the outer buoyancy columns extend just above
the sea surface; (e) floating a deck structure over the base, the
outer buoyancy columns, and the central columnar buoy; (f)
deballasting the outer columns to lift the deck structure; (g)
deballasting the central columnar buoyancy apparatus to raise it to
its upper position in which it engages the deck structure to form a
platform; (h) towing the platform to a second site at an
intermediate draft; (i) ballasting down the platform to an
operational draft; and (j) anchoring the platform to the
seabed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a simplified elevational view of a preferred embodiment
of the invention;
FIG. 1B is a cross-sectional view taken along line 1B--1B of FIG.
1A;
FIGS. 2A, 2B, and 2C are elevational views showing different types
of compliant guides used in the invention;
FIGS. 3A, 3B, 3C and 3D show different configurations for the buoy
used in the invention;
FIG. 4 shows a detailed view of the riser system and the single
buoy;
FIG. 5 is a diagrammatic view showing the creation of a restoring
moment in the buoy; and
FIGS. 6A to 6D show the different steps of the installation of the
platform, in accordance with the method of the invention.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1A and 1B show a deep draft semi-submersible platform 10
comprising a buoyant base 12 with a first moon pool 14 (which can
be circular, rectangular, etc.), four outer buoyant vertical
columns 16 (although any number greater than two can be used), a
production deck 18 supporting the process equipment, the quarters
and utilities, and a drilling or well deck 20, with its associated
equipment (if need be) and having a second moon pool 22. The deep
draft semi-submersible platform has a draft of at least 150 ft. (45
m), providing it with a low heave response, and low motion
responses to environmental loads (wind, waves and currents). These
motion characteristics allow the platform to support a vertical
riser system (Top Tensioned Risers), described in more detail
below. Alternatively, the deep draft semi-submersible platform 10
can be a self-installing platform or an extended draft platform, as
disclosed in U.S. Pat. No. 6,020,040. The deep draft
semi-submersible platform is anchored on the sea bed with mooring
lines (not shown), which may be either a taut leg mooring system or
conventional catenary mooring, to limit its horizontal offset.
The riser system comprises a plurality of vertical risers 24
supported by a riser buoyancy apparatus that is embodied as a
central columnar buoy 26 (which may comprise either a large single
buoyancy can or a multi-cellular buoyancy apparatus) received
within the floating platform 10. A novel feature of the present
invention is that the columnar buoy 26 is received in and guided
within the two moon pools 14, 22 of the floating platform 10. In
this way, the buoy 26 is guided at an upper location in the
production deck 20 and a lower location in the base 12, and is thus
restrained by the floating platform for horizontal and rotational
(about horizontal axes) movements. Furthermore, since the buoy 26
is guided within the moon pools 14, 22, the impact loads between
the floating platform and the buoy 26 due to wave and current
actions on the floating-platform are reduced.
The risers 24 extend from their respective wellheads 28 on the
seabed 30 to the well deck 20 located on top of the buoy 26. The
risers 24 enter the buoy 26 at its bottom or keel 32 through a
horizontal restraint apparatus that is described below in
connection with FIG. 4. The risers 24 are then attached to the top
of the buoy 26 where the well deck 20 is located. Surface trees
(not shown) on the well deck 20 are connected to the tops of the
risers 24, and the surface trees and jumpers (not shown) are used
to carry the petroleum product from the well deck 20 to the
production deck 18 on the work platform where the product will be
processed. In a specific example, the well deck 20 is supported
directly by the single buoy 26. However, as in prior art systems,
the well deck 20 can be supported by the floating platform itself,
being free to move up and down relative to the surface trees 34 and
the risers 24.
As can be seen in FIG. 1B, a lower plurality of buoy guides 36 (in
this example, four guides, but three or more can be used, depending
on the load to be absorbed by the guides) extends into the lower
moon pool 14 from the base 12. Preferably, these guides are
compliant. The lower buoy guides 36 significantly reduce the gap
between the buoy 26 and the base 12 within the lower moon pool 14
for further reducing the impact loads. A similar upper plurality of
compliant buoy guides 36 (not shown) extends into the upper moon
pool 22 from the production deck 18 to reduce the gap between the
buoy 26 and the production deck 18. As described more fully below,
each of the buoy guides 36 comprises a steel projection coated with
Teflon or polypropylene. Preferably, the buoy guides 36 are
configured and located to be in constant, uninterrupted contact
with the buoy 26. In order to do so, the buoy guides 36 must be
compliant enough to allow the installation of the central columnar
buoy 26, and also to allow the relative vertical motions between
the buoy 26 and the floating platform, while also accommodating any
buoy diameter variances from its nominal diameter due to
manufacturing tolerances. The guides 36 may include, at their free
ends, a wear pad mounted on a compliant support (an elastomeric
block or a leaf spring), as disclosed and claimed in
commonly-assigned U.S. Pat. No. 6,679,331, the disclosure of which
is incorporated herein by reference. As described in more detail
below, to further reduce the friction between the buoy 26 and the
guides 36, a wheel allowing vertical movement of the buoy 26 may
also be mounted on a compliant support.
With this arrangement, the present invention proposes to make the
single buoy 26 completely dependent from the deep draft
semi-submersible platform 10. The single buoy 26 will move with the
platform except for heave motions, and the interaction between the
buoy 26 and the platform will significantly ameliorate the motions
of the platform, as discussed below in connection with FIG. 5.
FIGS. 2A to 2C show different examples of compliant buoy guides 36.
FIG. 2A shows a standard compliant guide 36 comprising a wear pad
38 (preferably made of a suitable steel) with a contact surface
formed by a coating or layer of PTFE or polypropylene. The wear pad
38 is supported on the free end of a steel projection 40, the other
end of which is fixed to the base 12 or the production deck 18. In
between the steel projection 40 and the wear pad 38, a compliant
element 42 is arranged to allow the guide 36 to absorb impact loads
and to accommodate buoy diameter variances. The compliant element
42 preferably comprises one or more elastomeric blocks, as shown in
FIGS. 2A and 2B; alternatively it may comprise one or more leaf
springs (not shown). The stiffness of the compliant element is
selected, depending on the environmental conditions, and it may
comprise either a single stiffness compliant system (one grade of
elastomer or a constant stiffness leaf spring) or a multi-stiffness
compliant system in order to provide the guide with anon-linear
stiffness to absorb loads of different magnitudes (several grades
of elastomer, or leaf springs of several different stiffnesses) as
suggested in U.S. Pat. No. 6,679,331.
FIG. 2B shows an alternative guide 36', in which the wear pad is
replaced by a wheel and rail assembly. A wheel or roller 44 is
rotatably mounted in a pair of journals 46 (only one of which is
shown) supported at the free end of a steel projection 40' through
a compliant element 42'. The wheel 44 allows the vertical relative
motion between the platform and the buoy 26, and it further reduces
the friction between the two floating elements. Each wheel 44 rides
on a corresponding vertical rail 47 arranged on the outer surface
of the buoy 26. Another advantage of the wheel/rail assembly is
that it prevents rotation of the buoy 26 about its vertical axis.
The wheel/rail assembly may provide a steel-to-steel contact (as
friction is already reduced by the use of the wheel) or the wheel
44 and/or the rail 47 may be coated with PTFE or polypropylene.
FIG. 2C shows another embodiment for the guides (which can apply to
both alternatives described above). In this embodiment, the guide
comprises a guide module 48 riding on a horizontal rail 50 disposed
longitudinally along the upper surface of the base 12 of the work
platform 18, thereby allowing the module 48 to slide from a storage
position (out of contact with the buoy 26) to an operational
position (in contact with the buoy 26. The module 48 includes a
conventional locking mechanism (not shown) that can be operated by
a diver or a remote operating vehicle (underwater robot) (not
shown). The module 48 can be deployed, via a cable 54 and harness
56, from the platform using a crane (not shown) on the platform. To
this end, the module 48 is provided with one or more harness
attachment elements 58 on its upper surface. The module 48 is
installed on the rail 50, and then slid toward the buoy 26. The
module 48 is then locked into its operation position on the support
element 52 to secure it to the base or work platform when the
required preload is achieved. This arrangement simplifies the
installation of the buoy 26 without the risk of damage to the
compliant guides.
FIGS. 3A to 3D show different alternatives for the riser buoyancy
apparatus. The riser buoyancy apparatus may comprise a single buoy,
or multiple buoys closely spaced and connected to each other by
webs.
FIG. 3A shows a single buoy 26 having a central passage 60 to
receive a drilling riser or a tendon (not shown). Two moon pools 62
are arranged on either sides of the central passage 60. A plurality
of production riser passages 64 is arranged in the remaining
interior space of the buoy 26. In this arrangement, the risers pass
through the void compartments of the buoyancy apparatus, which may
require additional welding procedures to ensure sealing
efficiency.
FIG. 3B shows a single buoy 26' provided with a large center well
66. The center well 66 includes a plurality of riser passages 68
for the different risers, leaving enough room to receive a drilling
riser (not shown) in the center, or provide a moon pool for
lowering subsea hardware (not shown). In this embodiment, the
risers do not pass through the void compartments of the buoyancy
apparatus.
FIG. 3C shows a single buoy 26", wherein riser passages 70 are
arranged on the outer surface of the buoy 26". A center well 72 can
be arranged to act as a moon pool or to receive a drilling riser or
tendon (not shown).
FIG. 3D shows a multiple cell buoyancy apparatus 26'", comprising a
plurality of vertical outer tubular buoys 74, closely spaced and
connected to each other and to a central tubular buoy 76 by a
network of vertically-elongated webs 78. A plurality of risers 80
is arranged in the interstices defined between the tubular buoys
74, 76. If need be, the central buoy 76 can be designed to act as a
center well or to receive a drilling riser or tendon (not
shown).
The embodiment of FIG. 3D solves some problems inherent in the
single buoy embodiments. For example, to achieve a high degree of
compartmentalization, a single buoy must be sub-divided with a
large number of bulkheads, thereby increasing its cost of
manufacture. Furthermore, because the risers and/or tendons pass
through the buoy, the intersections between the risers and the buoy
and its bulkheads must be sealed by welding, using a heavy welding
procedure. In the embodiment shown in FIG. 3D, by contrast, the
vertically restrained buoyancy apparatus 26'" comprises an assembly
of a plurality of vertical tubular buoys 74, 76, closely spaced and
connected together by the vertically-elongated webs 78. This
arrangement achieves a high degree of compartmentalization with few
bulkheads and thus at a reduced cost. Furthermore, the risers can
be arranged around the exteriors of the tubular buoys 74, 76 (i.e.
in the interstices defined between them), and will therefore not
have to pass through the buoyancy compartments, thereby avoiding
the need to take further actions to ensure effective sealing.
In each of the buoyancy apparatus alternatives described above,
wear pads or rails 82 can be arranged on the outer periphery of the
buoy at the level of the guide apparatus to reduce friction.
FIG. 4 shows one way to horizontally restrain the riser 24 in the
lower part of the buoy 26. As will be explained below, it is an
important feature of the invention that at least the lower portion
of the riser 24 is horizontally restrained by the buoyancy
apparatus. (Alternatively, the riser 24 and the buoyancy apparatus
may be attached to each other). The riser 24 is received in a
vertical passage 84 disposed through the buoy 26, or in a stem (not
shown) connected to the buoyancy apparatus. The riser 24 is
attached to the top surface of the buoyancy apparatus and it is
guided in the lower part through a keel joint, so that the riser 24
is substantially in contact with the buoy passage 84 or stem, so
that loads (weight) of the risers will be transmitted to the
buoyancy apparatus through this keel joint. In this specific
example, the keel joint comprises two outwardly-tapered (radially
thickened) conjoined riser sections 86 to increase the section
modulus of the riser 24 in this area, and a ball wear insert 88, at
the juncture of the tapered riser sections 86. The ball wear insert
88 is able to move up and down in the passage 84, and it allows
some flexion about the keel joint, so that bending loads due to
platform motions will be absorbed by the keel joint.
FIG. 5 is a schematic drawing showing how the present invention
improves the pitch motion of the deep draft semi-submersible
platform. One of the advantages of the present invention is that,
because the buoy 26 is guided at two vertically spaced locations,
the contact loads between the buoy and the platform while the deep
draft semi-submersible platform is pitching (rotation around the
horizontal axis), create a restoring moment that reduces the pitch
motion of the platform. FIG. 5 shows the buoy 26 and its
environment (guides) when the platform pitches at a pitch angle
.alpha.. The buoyancy of the buoy 26 provides an uplift force (U)
which applies at the center of gravity (CG) of the buoy 26. The
weight of the riser (W.sub.R), because the risers are at least in
contact with the lower part of the buoy 26, will apply at the lower
part of the buoy. As the buoy 26 is pitching, the application
points of these forces are horizontally offset, and consequently
the horizontal resulting forces (Ux and W.sub.RX) in the oblique
two dimensional planes (defined by the longitudinal axis of the
buoy when tilting) are opposed. Because the buoy 26 is guided in
upper and lower locations, the buoy is restrained in rotation by
the platform, and the contact loads in the upper and lower guides
will correspond to the horizontal resulting forces and create a
moment. Since the weight of the risers is borne by to the lower
part of the buoy, the created moment opposes the pitching motion of
the platform and thus reduces the pitch angle .alpha.. The
restoring moment is proportional to the uplift force of the buoy.
Calculations have shown that the present invention can result in a
20% to 60% reduction in the pitch motion of the platform.
It is important to note that if the weight of the riser is borne at
the top of the buoy, the resulting moment will increase the pitch
angle and thus deteriorate the motion of the platform.
FIGS. 6A to 6D show the different steps of the installation method
of the platform of the present invention. In accordance with this
method, the central columnar buoy 26 is provided with an upper stop
assembly 90 and a lower stop assembly 92 to limit the vertical
motion of the buoy between upper and lower positions when it is
ballasted up or down, respectively, during installation, as
described below.
As shown in FIG. 6A, an assembly is provided that comprises a
buoyant base 12, plural vertical outer buoyancy columns 16, and a
central columnar buoyancy apparatus 26. The central buoyancy
apparatus 26 centrally located in the base 12, and it is movable
vertically relative to the base 12 from an upper position to a
lower position. The assembly is towed at a shallow draft to a first
site with the central columnar buoyancy apparatus 26 in its upper
position. Upon arrival at the first site, as shown in FIG. 6B, the
center columnar buoyancy apparatus 26 is ballasted down through the
base 12 to its lower position, at which the lower stop assembly 92
abuts against the base 12.
Then, as shown in FIG. 6C, the base 12 is ballasted down to a first
depth such that the outer buoyancy columns 16 extend just above the
sea surface. A deck structure (production deck 18 and well deck
20), supported by a deck barge 94, is floated over the base 12, the
central buoyancy apparatus 26, and the outer buoyancy columns 16.
At this stage, the well deck is seated on a rim 96 surrounding the
upper moon pool 22. The outer buoyancy columns 16 are the
deballasted to lift the deck structure off the barge 94, which is
then removed, and the production deck 18 is secured to the outer
columns 16, thereby forming a platform 10. Finally, as shown in
FIG. 6D, the central columnar buoyancy apparatus 26 is deballasted
to raise it to its upper operating position, at which the upper
stop assembly 90 abuts against the underside of the deck structure.
As the central buoyancy apparatus 26 rises to its operating
position, it lifts the well deck 20 off the upper moon pool rim 96
to the raised operational position of the well deck 20.
With the platform in the configuration shown in FIG. 6D, it is
towed to a second (operational) site at an intermediate draft. The
entire platform is then ballasted down to an operational draft and
anchored to the seabed by conventional anchoring means, such as a
taut leg mooring system.
The central buoyancy apparatus will not be protected by a center
well in the splash zone, and will be subjected to wave and current
action, which can lead to VIV problems. Because the diameter of the
vertically restrained central buoyancy apparatus 26 is large
compared to the diameter of a typical riser, the tension of the
riser system can be designed to limit this VIV problem. If need be,
VIV strakes can be arranged on the outer periphery of the buoy 26.
However only one set of VIV strakes will be required, and not one
set for each riser.
It will be appreciated that the central buoyancy apparatus 26 can
be vertically restrained by the risers themselves or by a central
tendon (not shown). The buoyancy apparatus 26 supports the well
deck 20, and high-pressure flexible jumpers (not shown) are used
for connection to the production deck 18. Alternatively, the well
deck 20 may include a manifold (not shown) to which the petroleum
will be carried and pressure choked down, and a low-pressure jumper
(not shown) can be used to carry the petroleum product to the
production deck. The buoyancy apparatus 26 can also support the
drilling deck. Furthermore, the risers and/or tendons will act
together as a single riser system.
* * * * *